Definitely yeast. My last case of thrush came on BECAUSE I had just taken abx for strep throat. Abx kill the good bacteria too, letting the yeast take over your body. I take a small 1 oz spray bottle (found at Walmart in the travel soap section) and fill it halfway with water and 15-20 drops of GSE in it. I spray my nipples and let them air dry.

Yeast loves dark moist places. So if your nursing pads get at all wet, change them. Do not leave wet ones on. Air out as much as you can. Go topless if possible. (I can't do this now because I have a 7 year old boy at home!) I've also taken a "yeast cleanse" that has GSE and TTO and a bunch of other things in it. It says not to take if you're nursing, but my midwife said it was fine. Also, cut out sugar and refined carbs for a while if you can.